Well, there can be two answers. So, plus 3 other friends (4 people total) are sharing stickers.
To answer this, all you need to do is count by four.
4 is not greater than 7 so can not be the answer
4+4 or 8 IS greater than 7 and less than 13 so it can be the answer
4+4+4 or 12 IS ALSO GREATER THAN 7 and less than 13 so it can be the answer
4+4+4+4 is 16 and is greater than 13 so it cannot be the answer
SO it is either 8 or 12 stickers